a) Field of the Invention
The present invention concerns polymer electrolyte batteries, for example, polymer electrolyte generators having a potassium salt enabling to stabilize the performances and service life of the battery. More specifically, the invention relates to rechargeable lithium generators containing a potassium salt which is distributed in the cathode, in the polymer electrolyte, or both of these at the same time. In particular, the present invention is directed to rechargeable electrochemical generators in which the potassium ions introduced in the form of additives in the cathodes as well as in the polymer electrolyte define an in situ treatment which lasts the entire service life of the generator so as to improve performances during cycling, for example, in terms of energy and power. The present invention also concerns potassium base additives distributed in at least one, and even two, of the components of a rechargeable lithium electrochemical generator, preferably the polymer electrolyte and the composite cathode bound to a polymer, in which the effect is to stabilize the energy and power performances during cycling.
The invention also describes preferred means for introducing potassium into the generator by either one of its components and describes how the potassium is distributed in more than one component so as to optimize the operation of the electrodes during cycling. The additive has the beneficial effect of maintaining the morphology of the lithium anode during cycling and to optimize the physical properties of the cathode during cycling.
b) Description of Prior Art
The life of a battery is dependent on many factors including the reversibility of the electrochemical processes at both electrodes. The addition of alkali earth or transition metals to the active material of the cathodes of lithium batteries is known and is used generally to stabilize or optimize the insertion cathodes (FR 2,616,013; U.S. Pat. No. 5,013,620; U.S. Pat. No. 5,114,809; FR 2,573,250). The additives used are generally intended to stabilize the insertion structures and sometimes to optimize the number of sites available in the host structure (WO 91/02383; U.S. Pat. No. 4,668,594). In some cases, the additives are also intended to increase the electronic conductivity of the insertion materials (U.S. Pat. No. 4,965,151; JP 89/15317; JP 89/67063; U.S. Pat. No. 5,114,811; U.S. Pat. No. 5,147,737). In most of the cases known, the addition metals are integrated in the host structure and are present at relatively high rates which vary between 1% and 50% with respect to the main transition metal. These additives are generally immobilized in the host insertion structure and are not diffused in the other components of the generator, for example, the electrolyte and the anode. In the case where the addition metals would be soluble in the electrolyte, they would be reduced with metallic lithium and could not remain in equilibrium in the generator. Moreover, in Applicant's view, no example of additive which is present in more than one component of a lithium generator has been described up to now.
As a matter of fact, a few of these metals are chemically compatible with a lithium anode and are capable of coexisting with the lithium salts which are in solution in the electrolyte of the generator. Potassium, with magnesium is one of the only metals which are not reduced (thermodynamically and kinetically) by lithium in aprotic media and therefore constitutes a unique material to carry out the present invention.
The utilization of polymer electrolytes with mixed alkali cation has been mentioned during conductivity measurements (ACFAS 1993) and for the constitution of vehicular conduction electrolytes (cf. U.S. Pat. No. 5,350,646). None of these cases mention an equilibrium between the mixed cations of the electrolyte and the materials of the electrode or a beneficial effect on the cycling of the generator or on the lithium anode.
It is an object of the present invention to provide for a beneficial effect noted on the stabilization of the material of the cathode V.sub.2 O.sub.5 which is presently used in the technology of polymer electrolyte batteries and simultaneously to provide an improvement to the reversibility of the dissolution-redeposition of the lithium anode and its morphology in order to improve the performances and service life of the battery.